The use of non-dispersive infrared spectroscopy to detect hydrocarbon gases is well established. It essentially involves transmitting infrared radiation along a path in an area being monitored; the wavelength of the infrared radiation is chosen so that it is absorbed by the gas of interest (hereafter called the “target gas”), but not substantially absorbed by other gases in the atmosphere of the area being monitored. The intensity of the radiation that has passed along the path in the area being monitored is measured and the attenuation in the intensity of the radiation gives a measure of the amount of target gas in the monitored area.
However, factors other than absorption by the target gas also attenuate the infrared radiation, including obscuration of the detecting beam, atmospheric scattering of the radiation, contamination of the lens surfaces, e.g. by dirt or condensation, and ageing of components. The reliability of infrared gas detectors is significantly improved by the use of a reference wavelength band; such a reference is usually infrared radiation at a different wavelength which ideally is a wavelength at which the target gas does not exhibit significant absorption. Radiation at more than one reference wavelength may be used; likewise more than one target wavelength may be used. Measuring the ratio between the signal obtained at the wavelength(s) where the target gas does absorb (the “sample” wavelength(s)) and the signal obtained at the wavelength(s) where the target gas does not significantly absorb (the “reference” wavelength(s)) more accurately measures the attenuation caused by environmental conditions because in most cases the signal at the reference wavelength(s) and the signal at the sample wavelength(s) will both be similarly affected by effects (other than the presence of target gas) that attenuate the radiation.
Usually, there are separate transmitter and receiver units at opposite ends of a straight beam path. Alternatively, the source and receiver are combined, and the beam bounced off a retroreflector at the far end of the measurement path. For portable use, detectors have also been made which use a remote object having suitable natural albedo in place of the retroreflector. The presence of a chosen gas (or class of gases) is detected from its absorption of a suitable infrared wavelength in the beam. Rain, fog etc. in the measurement path can also reduce the strength of the received signal, so it is usual to make a simultaneous measurement at one or more reference wavelengths. The quantity of gas intercepted by the beam is then inferred from the ratio changes of the signal losses at the measurement and reference wavelengths. The calculation is typically carried out by a microprocessor which also carries out various checks to validate the measurement and prevent false alarms.
Current open path gas detectors use an imaging optical system including a beam splitter to provide a signal for each detector where each detector has a dedicated bandpass interference filter to allow the appropriate wavelength to be transmitted to the intended detector. In this arrangement using a beam splitter, signal loss is 50% in each channel since half of the beam is sent to each detector. This arrangement is sensitive to slight misalignment between the dual optical channels that can lead to non-uniform images on the two detectors and erroneous gas determinations. Even small changes in alignment (<0.1 degree) or partial beam blockage) between the optical transmitter and receiver can lead to incorrect performance since the radiation cannot be accurately received on the misaligned photodiodes.
What is needed is an improved open path gas detection system that allows operation notwithstanding larger misalignment of the transmitter and the receiver and partial beam blockage of the transmitter and the receiver.